The higher the air temperature is, the higher the saturated humidity becomes. In medicine, the relative humidity is closely associated with human respiration. At a relative humidity of about 45 to 50 percent, the human body feels most comfortable since oxygen easily passes through the alveoli into the blood. When both the temperature and relative humidity are high, the human body feels sultry and uncomfortable as the thermoregulatory function of perspiration is prone to be influenced. In an environment with insufficient humidity, the human skin may feel dry and itchy, and the mucous membranes tend to get irritated. In addition, the relative humidity is very important to museums, libraries, manufacturing industry, building industry, wine industry, etc. A high humidity environment tends to breed worms, mildew, termites and bacteria, etc. A low humidity environment is likely to result in the moisture content of a material being too low, thus leading to damage such as the variation of stress, embrittlement, cracking and warping in the material.
However, conventional humidity regulating apparatuses, such as a humidifier merely capable of increasing the relative humidity and a dehumidifier merely capable of decreasing the relative humidity, can not simultaneously serve both of the functions for increasing and decreasing the relative humidity, so they lack practicality.